Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers, and many other products. A die-level packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The microelectronic die generally has an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The bond-pads are coupled to terminals on the interposer substrate or lead frame. The interposer substrate can also include ball-pads coupled to the terminals by traces in a dielectric material. An array of solder balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid arrays are generally higher grade packages that have lower profiles and higher pin counts than conventional chip packages that use a lead frame.
Packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching individual dies to an individual interposer substrate, (d) wire-bonding the bond-pads to the terminals of the interposer substrate, and (e) encapsulating the dies with a molding compound. It is time consuming and expensive to mount individual dies to individual interposer substrates. Also, as the demand for higher pin counts and smaller packages increases, it becomes more difficult to (a) form robust wire-bonds that can withstand the forces involved in molding processes and (b) accurately form other components of die-level packaged devices. Therefore, packaging processes have become a significant factor in producing semiconductor and other microelectronic devices.
Another process for packaging microelectronic devices is wafer-level packaging. In wafer-level packaging, a plurality of microelectronic dies are formed on a wafer and then a redistribution layer is formed on top of the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and traces coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding microelectronic die, and the ball-pads in each array are coupled to corresponding bond-pads on the die by the traces in the redistribution layer. After forming the redistribution layer on the wafer, a stenciling machine deposits discrete blocks of solder paste onto the ball-pads of the redistribution layer. The solder paste is then reflowed to form solder balls or solder bumps on the ball-pads. After formation of the solder balls on the ball-pads, the wafer can be cut to singulate the dies. Microelectronic devices packaged at the wafer level can have high pin counts in a small area, but they are not as robust as devices packaged at the die level.
Electronic products require packaged microelectronic devices to have an extremely high density of components in a very limited space. For example, the space available for memory devices, processors, displays, and other microelectronic components is quite limited in cell phones, PDAs, portable computers, and many other products. As such, there is a strong drive to reduce the height of the packaged microelectronic device and the surface area or “footprint” of the microelectronic device on a printed circuit board. Reducing the size of the microelectronic device is difficult because high-performance microelectronic devices generally have more bond-pads, which result in larger ball-grid arrays and thus larger footprints.
Image sensor dies present additional packaging problems. Image sensor dies include an active area that is responsive to light or other electromagnetic radiation. In packaging, it is important to form a cover that protects the active area without obstructing or distorting the passage of light or other electromagnetic radiation to the active area. One existing method for packaging an image sensor die includes placing the die in a recess of a ceramic substrate and attaching a glass window to the substrate over the active area. The window is hermetically sealed to the substrate to enclose the image sensor die. A vacuum pump typically removes air from the gap between the image sensor die and the glass window. An inert gas can then be injected into the gap between the image sensor die and the glass window.
U.S. Pat. No. 6,266,197 discloses another existing method for packaging image sensor dies by attaching and wire-bonding an array of image sensor dies to a carrier substrate. Next, a molded window array is placed over the image sensor dies. The molded window array includes sidewalls that are attached to the carrier substrate between the wire-bonds of adjacent dies and windows that extend between the sidewalls over corresponding dies. The substrate and the attached window array are then cut to form a plurality of individual image sensor packages.
One drawback of packaging image sensor dies in accordance with the above-mentioned methods is that the packaged image sensor dies are relatively bulky and, accordingly, use more space on a circuit board or other external device.